1. Field of the Invention
This invention relates to a non-contact type defect detecting device for detecting whether or not parts or elements of apparatus or systems are accurately located in or removed from some predetermined positions, and more particularly to a non-contact type defect detecting device for detecting whether or not an element or a part of an apparatus or system is precisely positioned at or completely removed from a predetermined position at a certain time of an operating cycle of the apparatus or system. This invention is particularly applicable to a watching device in a plastic injection molding system for watching the incomplete separation of the molded article from the mold.
2. Description of the Prior Art
Apparatus or systems generally include movable parts or elements which are to be moved relative to other parts. In order to precisely operate the apparatus or systems, such movable parts or elements must be moved in order. If the movable parts or elements are not precisely positioned at a predetermined position with respect to other elements where the elements should be positioned at a certain time of an operating cycle of the apparatus or systems, the apparatus or systems would get out of order, and sometimes would be damaged. In order to watch whether or not the movable parts are moved in order, or in order to ensure that the movable parts are precisely moved in response to movement of other parts, there have conventionally been used limit switches, micro switches or photoelectric switches which are disposed at the position to see if the movable parts are moved to or from the predetermined position.
However, the limit switch or the micro switch has a drawback in that they must be located close to the apparatus and put into direct contact with the movable parts or elements (hereinafter simply referred to as movable parts). Accordingly, the limit switch or the micro switch cannot be used unless there is a space for the switch to be located in contact with the movable parts and the switch is able to be in direct contact with them. When these kinds of switches cannot be used, photoelectric switches can be used. However, the photoelectric switches must be accurately located with respect to the movable parts so that they are surely actuated in response to the movement of the movable parts. Accordingly, there is required a long time and a skill when the location of the photoelectric switches is changed to change the operational mode of the apparatus or system.
In a plastic injection molding machine, a molten plastic material is injected under pressure into a closed mold cavity formed between a movable mold half and a fixed mold half to fill the cavity. The movable half is moved away from the fixed half to open the mold after the plastic material is filled and cooled in the cavity, and the hardened article is removed from the cavity. Then, the movable half is moved toward the fixed half to be pressed thereagainst to form the closed mold cavity again. In an automatic operational mode of the plastic injection molding machine, the hardened article is automatically ejected from the movable mold half by means of ejector pins and falls by its gravity. However it often happens that the article or a part of the article does not fall from the movable half sticking thereto or the article remains in the fixed half being grasped thereby. If the mold halves are closed with the hardened article left therebetween, the mold halves would be damaged since the article is normally displaced from the cavity and the mold halves are pressed against each other with an extremely high pressure.
Accordingly, it is generally required to use a mold watching device for watching whether the hardened plastic material is left in the mold. When the plastic material is left in the mold, the mold watching device gives a signal to the injection molding machine so as not to close the mold.
As the watching device, there have conventionally been used a weight scale type mold watching device which confirms that whole of the hardened plastic article including products, a runner, a sprue and the like has fallen from the mold by comparing the overall weight of the plastic article falling onto the scale with a predetermined standard weight. The standard weight is preset in accordance with the overall weight of the plastic article which should fall from the mold. When the overall weight of the plastic article which has fallen onto the scale is smaller than the standard weight, the watching device indicates that a part of the molded plastic article remains in the mold.
However, the weight scale type mold watching device is disadvantageous in that it requires a long time to carry out the determination with a sufficient accuracy when the mold includes a number of cavities to obtain a number of products of very small weight in one shot, since the weight of each product may ordinarily change in every shot. Further, even if the mold has a single cavity, the watching device would malfunction when a part of the plastic article left in the mold is very small in weight relative to its overall weight. Even such a small amount of plastic material could damage the mold.
Furthermore, the weight scale type mold watching device is usually located below the mold. The space below the mold is generally limited. Therefore, when the space is occupied by the watching device, it is difficult to provide a transferring means such as a conveyor belt for transferring the article falling from the mold to another processing station.
Further, plastic injection molds generally have a plurality of fixed pins and movable pins. Such fixed pins as well as the movable pins could be accidentally bent during a number of operation cycles of the injection machine. The fixed pins are generally received in recesses or holes provided in the corresponding mold half. Therefore, when the fixed pin is bent to get out of alignment with the corresponding recess or hole, the fixed pin would be broken when the mold is closed. The movable pins are generally projected from the inner surface of one mold half when the mold is opened and are retracted from the surface being pressed by the surface of the other mold half upon closure of the mold. When the movable pins are bent, they could not be retracted and would be damaged upon closure of the mold. At the same time, when the fixed or movable pins are broken, they could possibly scratch the surface of the cavity.
Needless to say, said weight scale type mold watching device cannot see whether the fixed or movable pins are in the right position thereof.